An antireflection technique which has been receiving attention in recent years is forming over a substrate surface a very small uneven pattern in which the interval of recessed portions or raised portions is not more than the wavelength of visible light (λ=380 nm to 780 nm). See Patent Documents 1 to 4. The two-dimensional size of a raised portion of an uneven pattern which performs an antireflection function is not less than 10 nm and less than 500 nm.
This method utilizes the principles of a so-called moth-eye structure. The refractive index for light that is incident on the substrate is continuously changed along the depth direction of the recessed portions or raised portions, from the refractive index of a medium on which the light is incident to the refractive index of the substrate, whereby reflection of a wavelength band that is subject to antireflection is prevented.
The moth-eye structure is advantageous in that it is capable of performing an antireflection function with small incident angle dependence over a wide wavelength band, as well as that it is applicable to a number of materials, and that an uneven pattern can be directly formed in a substrate. As such, a high-performance antireflection film (or antireflection surface) can be provided at a low cost.
As the method of forming a moth-eye structure, using an anodized porous alumina layer which is obtained by means of anodization of aluminum has been receiving attention (Patent Documents 2 to 4).
Now, the anodized porous alumina layer which is obtained by means of anodization of aluminum is briefly described. Conventionally, a method of forming a porous structure by means of anodization has been receiving attention as a simple method for making nanometer-scale micropores (very small recessed portions) in the shape of a circular column in a regular arrangement. An aluminum base is immersed in an acidic electrolytic solution of sulfuric acid, oxalic acid, phosphoric acid, or the like, or an alkaline electrolytic solution, and this is used as an anode in application of a voltage, which causes oxidation and dissolution. The oxidation and the dissolution concurrently advance over a surface of the aluminum base to form an oxide film which has micropores over its surface. The micropores, which are in the shape of a circular column, are oriented vertical to the oxide film and exhibit a self-organized regularity under certain conditions (voltage, electrolyte type, temperature, etc.). Thus, this anodized porous alumina layer is expected to be applied to a wide variety of functional materials.
A porous alumina layer formed under specific conditions includes cells in the shape of a generally regular hexagon which are in a closest packed two-dimensional arrangement when seen in a direction perpendicular to the film surface. Each of the cells has a micropore at its center. The arrangement of the micropores is periodic. The cells are formed as a result of local dissolution and growth of a coating. The dissolution and growth of the coating concurrently advance at the bottom of the micropores which is referred to as a barrier layer. As known, the size of the cells, i.e., the interval between adjacent micropores (the distance between the centers), is approximately twice the thickness of the barrier layer, and is approximately proportional to the voltage that is applied during the anodization. It is also known that the diameter of the micropores depends on the type, concentration, temperature, etc., of the electrolytic solution but is, usually, about ⅓ of the size of the cells (the length of the longest diagonal of the cell when seen in a direction vertical to the film surface). Such micropores of the porous alumina may constitute an arrangement which has a high regularity (periodicity) under specific conditions, an arrangement with a regularity degraded to some extent depending on the conditions, or an irregular (non-periodic) arrangement.
Patent Document 2 discloses a method of producing an antireflection film (antireflection surface) with the use of a stamper which has an anodized porous alumina film over its surface.
Patent Document 3 discloses the technique of forming tapered recesses with continuously changing pore diameters by repeating anodization of aluminum and a pore diameter increasing process.
The applicant of the present application discloses, in Patent Document 4, the technique of forming an antireflection film with the use of an alumina layer in which very small recessed portions have stepped lateral surfaces.
Utilizing an anodized porous aluminum film in this way can facilitate the manufacture of a mold which is used for formation of a moth-eye structure over a surface (hereinafter, “moth-eye mold”). In particular, as described in Patent Documents 2 and 4, when the surface of the anodized aluminum film as formed is used as a mold without any modification, a large effect of reducing the manufacturing cost is achieved. The structure of the surface of a moth-eye mold which is capable of forming a moth-eye structure is herein referred to as “inverted moth-eye structure”.
On the other hand, as described in Patent Documents 5 to 8, a method for manufacturing an anodized porous alumina film (self-supporting film) which has through holes has been researched.
Patent Document 5 discloses a method which includes anodizing an aluminum layer that is in tight contact with an aluminum base to form a porous alumina layer which has a barrier layer on the base side and a porous layer only on the aluminum layer side, and separating only the porous layer portion as a self-supporting film.
Patent Documents 6 to 8 disclose a method for obtaining a porous alumina layer by dissolving away an aluminum base remaining on the underlayer of a porous alumina layer after anodization.
The entire disclosures of Patent Documents 1, 2, and 4 to 8 are incorporated herein by reference.